High requirements are made of glasses for applications as substrates in liquid-crystal flat-panel display technology, for example in TN (twisted nematic)/STN (supertwisted nematic) displays, active matrix liquid crystal displays (AMLCDs), thin film transistors (TFTs) or plasma addressed liquid crystals (PALCs). Besides high thermal shock resistance and good resistance to the aggressive chemicals employed in the production process for flat-panel screens, the glasses should have high transparency over a broad spectral range (VIS, UV) and low density in order to save weight. Use as substrate material for integrated semiconductor circuits, for example in TFT displays ("chip on glass") in addition requires thermal matching to the thin-film material a- or polysilicon (.alpha..sub.20/300 =3.7.times.10.sup.-6 /K) and the absence of alkali metal ions. Sodium oxide amounts resulting from production of more than 1500 ppm cannot be tolerated in view of the generally "poisoning" action due to diffusion of Na.sup..fwdarw. into the semiconductor layer.
It should be possible to produce suitable glasses economically on a large industrial scale in adequate quality (no bubbles, knots, inclusions), for example in a float plant or by the drawing method. In particular, the production of thin. (&lt;1 mm) streak-free substrates with low surface undulation by the drawing process requires high devitrification stability of the glasses. Compaction of the substrate during production, which has a disadvantageous effect on the semiconductor microstructure, can be countered by setting a suitable temperature-dependent viscosity characteristic line of the glass: with respect to thermal process and shape stability, it should have a high strain point (SP; temperature at a viscosity of 10.sup.14.7 dPas), ideally above 700.degree. C., or a high glass transition temperature T.sub.g, i.e. T.sub.g &gt;720.degree. C., while on the other hand not having excessively high melting and working (V.sub.A) points, i.e. a V.sub.A of .ltoreq.1330.degree. C. Furthermore, a low density of the glasses is desired in order to keep the overall weight of the display low, in particular in the case of large screen formats.
The requirements of glass substrates for liquid crystal display (LCD) technology are also described in "Glass substrates for AMLCD applications: properties and implications" by J C Lapp, SPIE Proceedings, Vol. 3014, Invited paper (1997).
In principle, corresponding requirements are made of glasses for substrates in thin-film photovoltaics, especially based on microcrystalline silicon (.mu.c-Si).
An essential prerequisite for the commercial success of thin-film photovoltaics against high-cost solar technology based on crystalline Si wafers is the presence of inexpensive high-temperature-resistant substrates.
At present, two different coating methods are known for the production of .mu.c-Si solar cells. A process which has proven particularly favorable with respect to high deposition rates is a high-temperature chemical vapor deposition (CVD) process using inexpensive trichlorosilane as Si source. This process requires the heating of a suitable substrate to 1000.degree. C. or above. The only suitable substrates are then comparatively expensive ceramics, graphite, silicon or similar materials. Use of transparent glass-ceramics has also been discussed in the literature (L. R. Pinckney: "Transparent, High Strain Point Glass-Ceramics", Proc. 18th Intern. Conf. Glass, San Francisco; Amer. Ceram. Soc., Ohio, 1998, and L. R. Pinckney, G. H. Beall: "Nanocrystalline Non-Alkali Glass-Ceramics", J. NonCryst. Solids 219 (1997)). Efficiencies achieved on small areas by the high temperature CVD process are currently about 11%.
As an alternative to the high-temperature approach, low temperature Si deposition processes have been developed which allow the use of the less expensive substrate material glass. One possibility here is the deposition of amorphous silicon at low temperatures of up to 300.degree. C. and, in a subsequent step, the recrystallization of the layers using laser or zone-melting methods. In order to prevent deformation of the glass plate at the temperatures prevailing in the conditioning process, a special glass with very high heat resistance which is matched thermally to silicon is required. These glasses suitably have a glass transition temperature, Tof at least 750.degree. C. As a consequence of the tendency to change over from a-Si to poly-Si coatings, the highest possible heat resistance of the substrate is also desired for substrates for TFT display technology.
The current development of .mu.c-Si technology is moving in the direction of a substrate concept, i.e. the substrate material forms the basis of the solar cells and is opaque to the incident light. However, a development towards a less expensive superstrate arrangement (light passes through the substrate material, no cover glass necessary) is not excluded. In order to achieve high efficiencies, high transparency of the glass in the VIS/UV is then necessary, which means that the use of semi-transparent glass-ceramics, besides the above-mentioned cost reasons, proves to be disadvantageous.
The said requirement profile is satisfied most closely by alkaline earth metal aluminoborosilicate glasses. However, the known display or solar-cell substrate glasses described in the following specifications still have disadvantages and do not meet the entire range of requirements.
Numerous specifications describe glasses having relatively high B.sub.2 O.sub.3 contents, for example DE 196 01 922 A, JP 58-120 535 A, JP 60-141 642 A, JP 8-295 530 A, JP 9-169 538 A, JP 10-59 741 A, JP 10-722 37 A, EP 714 862 A1, EP 341 313 B1, and U.S. Pat. No. 5,374,595. These glasses do not have the requisite high glass transition temperatures or strain points.
The same applies to the low-SiO.sub.2 glasses of JP 61-132 536 A and to the glasses of DE 197 39 912 Cl containing a maximum of 60% by weight of SiO.sub.2 and at least 6.5% by weight of B.sub.2 O.sub.3.
By contrast, B.sub.2 O.sub.3 -free glasses are described in U.S. Pat. No. 4,607,016, JP 1-126 239 A, JP 61-236 631 A and JP 61-261 232 A. Owing to the freedom from B.sub.2 O.sub.3, the glasses have poor melting properties and tend toward devitrification. The glasses mentioned in WO 97/30001 likewise contain no B.sub.2 O.sub.3.
DE 44 30 710 C1 describes borosilicate glasses having low boric acid contents and high SiO.sub.2 contents (&gt;75% by weight), which results in them having high viscosity even at high temperatures and means that they can only be melted and fined at high cost. In addition, these glasses, having glass transition temperatures T.sub.g of from 500 to 600.degree. C., have only relatively low heat resistance.
DE 196 17 344 C1 and DE 196 03 698 C1 by the Applicant disclose alkali-free, tin-containing glasses having a coefficient of thermal expansion .alpha..sub.20/300 of about 3.7-10.sup.-6 /K and very good chemicals resistance. They are suitable for use in display technology. However, since they necessarily contain at least 1 or 2% by weight of the network modifier ZnO, they are not totally suitable, in particular for processing in a float plant.
Thus, the glasses of JP 61-295 256 A, which contain Pb and have a relatively high Zn content (.gtoreq.3.5% by weight) are likewise not very suitable for the float process, since coatings of ZnO and PbO or Pb can easily form on the glass surface in the reducing forming gas atmosphere due to evaporation and subsequent condensation at an excessively high concentration.
JP 3-164 445 A describes glass-ceramics having ZnO contents of .gtoreq.5% by weight for displays and solar cells. They have the desired high glass transition temperatures, but are poorly matched to .mu.c-Si with their thermal expansion of greater than 4.0.multidot.10.sup.-6 /K.
Further glass-ceramics which have a relatively high Zn content (.gtoreq.8% by weight) and in addition contain Pb are described in JP 1-208 343 A. These likewise have excessively high thermal expansion. Correspondingly, EP 168 189 A2 describes glass-ceramics containing .gtoreq.2% by weight of ZnO.
MgO-free glasses having high BaO and low Al.sub.2 O.sub.3 contents are described in DE 37 30 410 A1. The stated strain points are too low, and the density of the glasses will be disadvantageously high. The PCT applications WO 97/11919 and WO 97/11920 also describe alkali-free, MgO-free or low-MgO glass substrates.
The glasses of U.S. Pat. No. 5,116,788 have high contents of alkaline earth metal oxides with from 23 to 28 mol-% of RO, which means that they have excessively high thermal expansion for Si. The same applies to the alkali-free, high-temperature-resistant glasses which are used as lamp bulb glasses for halogen lamps. They are matched to Mo with respect to their thermal expansion. Examples which may be mentioned here are the glasses in the specifications U.S. Pat. No. 4,060,423(BaO 6-16% by weight), U.S. Pat. No. 3,978,362 (CaO 14-21% by weight) and EP 261 819 A1 (CaO 10.5-12.5% by weight; BaO 8.5-14% by weight).
EP 672 629 A2 and U.S. Pat. No. 5,508,237 describe aluminosilicate glasses for flat-panel displays. They have various composition ranges with various coefficients of thermal expansion. It is claimed that these glasses can be processed not only by the overflow fusion drawing process, but also by other processes for the production of flat glass. However, the glasses, in particular, which have a coefficient of thermal expansion which is matched to polycrystalline Si have very high working points V.sub.A, which make them unsuitable for the float process. As in the glasses described hitherto, the visual quality here is again not high, since no method for effective, in particular float-compatible fining is indicated. The fining agents Sb.sub.2 O.sub.3 and As.sub.2 O.sub.3 mentioned by way of example are unsuitable for the float process owing to their ease of reduction. The same applies to the optional glass components Ta.sub.2 O.sub.5 and Nb.sub.2 O.sub.5.
The specification JP 9-123 33 A, which relates to glasses for hard disks, describes compositions of Si.sub.2, Al.sub.2 O.sub.3, CaO and further optional components. The glasses listed have high alkaline earth metal contents and thus have high thermal expansion, which makes them unsuitable for use in LCD and PV technology. Their visual quality will probably also be inadequate.
The latter also applies to the relatively B.sub.2 O.sub.3 -rich glasses of JP 9-486 32 A, JP 9-156 953 A and WO 98/27019, in particular as no method for effective fining is indicated.
JP 9-263 421 A and JP 10-454 22 A describe alkali-free glasses which can be processed by the float method for use as substrates in flat-panel display systems. The glasses listed have very high working points and very high temperatures at a viscosity of 10.sup.2 dPas, which impairs their melting properties and makes inexpensive production impossible, since the requisite temperature range also means that very high requirements are made of the tank and distributor material with respect to corrosion resistance. The glasses of JP 10-454 22 A are free from TiO.sub.2, ZrO.sub.2 and CeO.sub.2. The density of the BaO-containing glasses is relatively high at p&gt;2.6 g/cm.sup.3 . The glasses of JP 9-263 421 A preferably contain no BaO and are free from TiO.sub.2, ZrO.sub.2, CeO.sub.2 and SnO.sub.2.